Single crystal phosphide production



P 23, 1968 A. ADDAMIANO 3,379,502

SINGLE CRYSTAL PHOSPHIDE PRODUCTION Filed Sept. 16, 1965 hnven tow" Arri o Addamiano b5 flaw/i His AFFEOTTwe wanna...

ABSTRACT OF THE DTSCLGSURE A method for the production of single crystal phosphides of gallium, aluminum or indium wherein a single continuous process is used to produce the phosphide and then to grow it into relatively large single crystals. The metal desired in the phosphide is reacted with zinc phosphide to produce metal phosphide, and zinc and any residual zinc phosphide are removed from the location of the reaction by volatilization. The metal desired in the phosphide is supplied in excess so that after volatilization of the zinc and zinc phosphide the reaction mixture can be taken to an elevated temperature for growth of the metal phosphide single crystals in a bath of the metal itself, thereby avoiding deleterious impurities.

The bath is then cooled and the excess metal removed such as by leaching.

This invention relates to a method for preparing single crystals of phosphides, and more particularly to the production of single crystal phosphides of the metals aluminum, gallium and indium.

Various methods for the preparation of phosphides of these metals are described and particular methods are claimed in Patent 3,008,805-Addamiano, assigned to the assignee of the present invention. Basically, that patent involves a method of reacting together in a neutral atmosphere at elevated temperatures gallium, aluminum or indium with zinc phosphide (Zn P or its equivalent ZnP to produce a phosphide of the selected metal according to the reaction Where Me is aluminum, gallium or indium. If stoichiometric amounts of the reactants are used, the metal is converted into the metal ph-osphide, and the by-product of the reaction, zinc, is removed from the reaction by vaporization. If an excess of ZilgPg is present, it can be removed from the reaction by sublimation after formation of the desired metal phosphide. These phosphides in single crystal form are in demand for several applications including diodes, transistors, and electroluminescent devices. The term single crystal herein refers to crystalline material in which at least one dimension of the crystal is greater than about 0.1 millimeters. However, the various methods known in the art for producing these phosphides generally produce them in the form of powders, or as polycrystalline aggregates, so that the preparation of single crystals usually involves one or more additional steps. These additional steps are normally expen sive and time-consuming and may be accompanied by hazards due to the high dissociation pressures of the phosphides.

One of the known methods for growing relatively large single crystals of such phosphides involves sealing small or irregular crystals of the metal phosphide along with an excess of the selected metal into a suitable reaction vessel and heating the vessel to dissolve the crystals in the excess of metal. Disadvantages of this process include the apparent necessity of high temperature, high pressure reaction in small sealed containers or bombs as States Patent well as the unfavorable economics of conducting the reaction in this manner.

It is an object of the present invention to provide a method for producing single crystals of aluminum, gallium and indium phosphides in which the phosphide can be initially formed and the single crystals grown in one continuous operation without cooling down or treating any intermediate products between the two parts of the process.

Another object of the invention is to provide such a process which is nonhazardous relative to the prior art and can be entirely programmed and automated.

Still another object is the provision of such a method in which it is not necessary to seal the reactants under vacuum in vitreous containers or in a high pressure vessel such as a bomb or autoclave.

Further objects and advantages of the invention will appear from the following detailed description of species thereof and from the accompanying drawing.

The sole figure of the drawing is a schematic representation partly in section of the inner operating parts of a furnace useful for practicing the present invention.

Briefly stated, the invention in one form provides a method for the preparation of single crystal phosphides of a metal selected from the group consisting of aluminum, gallium and indium wherein the phosphide is initially formed in a neutral atmosphere in a reaction chamber by reacting an excess of the selected metal with Zn P at a temperature in the range of 7001000 C. for a period of time sufiicient for the reaction to go essentially to completion. The required time will, of course, depend on the amounts and particle sizes of the constituents, among other factors. The reaction mixture is then raised to a temperature sufficient to eliminate any residual traces of free zinc by vaporization and of Zn P by sublimation, and then the reaction chamber is sealed at least moderately tightly, although a vacuumor gas-tight seal is not absolutely necessary. The reaction mixture is then raised to a temperature sufficient to dissolve the formed selected metal phosphide in the excess of the selected metal. For example, with gallium as the selected metal, a temperature of 1250 C. should be sufiicient to reach temperature and phase equilibrium. The reaction mixture can be stirred or rocked gently back and forth to enhance pro-gress towards equilibrium if desirable. Subsequently, the reaction mixture is cooled at a sufllciently slow rate such as about l10 C. per minute, or preferably 14 C. per minute down to a temperature at which substantially al of the selected metal phosphide has segregated from the bath as single crystals following a process of nucleation and growth. Further cooling can be at any desirable rate so long as unfavorable side effects such as quenching stresses from too fast cooling do not harm the product. The single crystals are then removed from the excess metal by conventional techniques such as acid dissolution of this metal. The resulting crystals can easily attain dimensions of a few millimeters.

I have found that an excess of the selected metal can be initially provided to the reaction mixture and the crystal-growing portion of the process can be carried out with only a moderately tight seal, avoiding the necessity for gas-tight vitreous seals or autoclave-type seals. In addition, I have discovered and demonstrated that the reaction can be carried out as described with quite satisfactory results.

Although the melting point of gallium, 29.8 C., is far below that of zinc, 419.5 C., gallium has a very high boiling point of 2237 C. as compared to 906 C. for zinc and a sublimation temperature of about 1050 C. for Zn l This makes it possible to drive oil the zinc and Zn P while retaining gallium. Likewise, aluminum melts at 660 C. and boils at 2450 C., and the respective temperatures for indium are 156.2 C. and 2000 C.

Turning now to the drawing, the sole figure is a schematic representation partly in section of a tube furnace adapted to the practice of the present invention. As illustrated, an outer tube 1 of a material such as quartz is provided to separate the outer parts of the furnace, not shown, from the inner working parts. The heating means may be provided either outside this outer tube 1, or at the reaction zone 3, which is a necked-down portion of an inner tube 2 of a material such as quartz, such as by the indicated resistance heating wires 5. If desired, heating means may be provided in both places to heat the entire furnace for the first part of the reaction and adjust the reaction zone to a higher temperature for the latter parts of the reaction.

The reaction mixture initially consisting essentially of an excess of the selected metal plus Zn P is placed in the reaction zone in a boat or other suitable vessel which might be made, for instance, of graphite, quartz or aluminum nitride. For the initial formation of the phosphides, a suitable gas such as argon can be passed through the inner tube 3 of the furnace to carry off zinc released and volatilized in the reaction, and later to remove the sublimated Zn P After the actual reaction has occurred and the unwanted zinc and Zn P have been removed from the reaction zone, plugs 6 may be pressed up against the suitably shaped shoulders of the necked-down portion 3, as indicated in dotted lines 7 for one of the plugs, to form a seal. The purpose of making this seal is to prevent excessive volatilization of phosphorus coming from dissociation of the phosphide. Thus, the seal may be only tight enough to prevent volatilization of such a great extent as would hamper the reaction. It is preferable to control the furnace temperature profile to minimize vaporization losses of phosphorus by making the end portions of tube 1 hotter than the reaction zone 3. As will be apparent, the plugs can be kept in the furnace continuously provided they are of the right size and shape to allow gas flow around them before they are moved into the sealing position 7. Likewise, the ends of the plugs 6 and the mating surfaces of the necked-down portions 3 may, for example, be spherical, conical or flat, although spherical surfaces are preferred for greater ease of operation. Temperature sensing and controlling means as known in the art may be used for operation of the invention in such a furnace as this.

As an example of the method of the invention, a mixture of 14 grams of gallium and 25.8 grams of Zn P was placed in a graphite boat in the reaction zone of a furnace such as described above. The reaction was carried out at 900 C. for about fifteen hours in an atmosphere of flowing argon to produce about 2 grams of GaP. Thus, about 12.6 grams of unreacted gallium were available for use as the solvent at the end of the reaction. The temperature was then raised to about 1100 C. for a few minutes to vaporize any residual traces of unreacted zinc and sublime any Zn P remaining, these being carried from the reaction zone by the flowing argon gas. The temperature was then brought to about 1250 C., the plugs 6 moved to the closing position, and the temperature kept at 1250 C. for at least thirty minutes to achieve complete dissolution of the GaP in the gallium liquid. Subsequently, the furnace was allowed to cool at a rate of from 14 C. per minute to about 900 C. at which temperature the electrical power input to the Cat 4 furnace'was turned off and the furnace was allowed to cool to room temperature. After removing the plugs, the boat was taken out of the furnace and the GaP crystals were extracted from the excess gallium by removing the gallium first mechanically and then by attack with hydrochloric acid. The crystals were well developed platelets, often of regular contour, hexagonally shaped, and as large as about 1-2 millimeters on a side (about 14 millimeters square), showing one shiny face, and, opposite to it, a relatively dull. one. This is an indication that the developed faces were (111) and (1T1) planes.

As a modification of this technique, the preparation can incorporate a dopant such as ZnO, ZnS, CdS, or others known in the art along with the selected metal and Zn P in order to produce crystals having desirably controlled electrical and other physical properties.

While specific examples have been given of the method of the invention, it will be understood that various changes, omissions and substitutions may be made within the true spirit and scope of the invention as defined in the appended claims.

What I claim as new and desire to secure 'by Letters Patent of the United States is:

1. The method of preparing single crystals of a posphide of a metal selected from the group consisting of aluminum, gallium, and indium, which comprises firing in a neutral atmosphere in a reaction chamber and at a temperature in a range of about 700l000 C. zinc phosphide and metal from said group for a time sufficient to form the selected metal phosphide, with said selected.

metal being supplied in excess of stoichiometric amounts, eliminating by vaporization and carrying away from the reaction chamber the zinc produce by the reaction and excess zinc phosphide, and then essentially closing off said reaction chamber and raising the temperature to a. level and for a time sufficient to dissolve the formed selected metal phosphide crystals in the excess of said selected metal, then slowly cooling to a temperature at which substantially all of the selected metal phosphide has precipitated from the melt.

2. The method of claim 1 wherein the selected metal is gallium, the reaction to form gallium phosphide is carried out at a temperature in the range of 700-900" C., zinc and excess Zn P are eliminated by vaporization at about 1100 C., and the dissolution of the formed gallium phosphide crystals is conducted at about 1250 C.,

followed by cooling at a rate of 110 C. per minute at least down to about 900 C.

3. The method of claim 1 in which the selected metal is aluminum.

4. The method of claim 1 in which the selected metal is indium.

References Cited UNITED STATES PATENTS 3,008,805 11/1961 Addamiano 23-404 3,009,780 11/1961 Stone 23204 2,871,100 l/l959 Guire et al. 2,921,905 1/ 1960 Hung-Chi Chang.

FOREIGN PATENTS 949,945 2/1964 Great Britain.

EDWARD J. MEROS, Primary Examiner.

' OSCAR R.VERTIZ, Examiner.

H. S. MILLER, AssistantExaminer. 

